1. Field of the Invention
The present invention relates to a high modulus pitch-based carbon fiber and a method for preparing the same. More specifically, the present invention relates to a pitch-based carbon fiber which has a high modulus of elasticity attained at a relatively low carbonization temperature. High modulus carbon fibers are used as composite materials with plastics, metals, carbon, ceramics and the like for light weight structural materials in aircraft, spacecraft, automobiles, and architecture, etc. and for high temperature materials such as those used in brake discs, rockets, etc.
2. Description of the Related Art
High tensile strength, intermediate modulus PAN (polyacrylonitrile) based-carbon fibers are prepared using polyacrylonitrile as the starting material and those prepared at a temperature above 2000.degree. C. may have a maximum Young's modulus of about 400 GPa. However, PAN-based carbon fibers, in addition to being unpreferably expensive starting materials, are a limited in increase of crystallinity (degree of graphitization) due to their non-graphitizable property, making it difficult to attain PAN-based carbon fibers having an extremely high modulus.
Pitch-based carbon fibers are very economical, due to their cheap starting materials, and those prepared from a petroleum liquid crystal pitch by carbonizing at temperatures near 3000.degree. C., referred as graphite fibers, exhibit an extremely high modulus of around 700 GPa (see, for example, U.S. Pat. No. 400518).
To improve the properties of pitch-based carbon fibers, such as tensile strength, Young's modulus, etc., there have been proposed, for example, carbon fibers having, in their cross section, structure oriented in the circumferential direction at an outer layer portion of the fiber and structure oriented in the radial direction or having a mozaic texture at an inner portion of the fiber (see Japanese Unexamined Patent Publication (Kokai) No. 59-53717), and carbon fibers having a radially oriented structure at an outer layer portion of the fiber and an onion-like texture at an inner core portion of the fiber, particularly when wishing to obtain an enhanced surface mechanical strength (Japanese Unexamined Patent Publication (Kokai) No. 60-239520).
Although, as mentioned above, carbon fibers having an extremely high modulus can be prepared by using a liquid crystal pitch, and some methods have been proposed for improving the properties of pitch-based carbon fibers, all of these methods require carbonization at a high temperature of near 3000.degree. C. to attain an extremely high modulus. Carbonization at such a high temperature not only requires high production cost, but also unpreferably decreases the tensile strength of the carbon fibers.